1957-58 Prefix "T" for Standardweight and Prefix
"A" for Sweet 16 numbers mixed, but range from
1-10900.
1958-1976 Ser. No. sequence changed to
include a one or two digit numeral followed by an
alpha character.

: FAQ When was my gun produced? All the Italian firearms and the foreign firearms produced in Countries that do not have a Proof House recognized in Italy, should be prooved at the Proof House of Gardone Val Trompia, before being sold. The Proof House performs a series of tests and controls of safety, to ascertain the operational qualities of the firearm and the absence of risks for the shooter. Proof of the fact that the controls of safety were performed is represented by stamps, deliberately put on the action or on the barrel. Among these, is (especially on rifled barrel guns) the date in which the arm was proofed. In the period included between 1922 and 1953, the date was indicated "in clear", with a stamp stating the entire year including the four figures.Starting from 1954, however, it began to indicate the year with a Roman number: 1954 corresponds to the X, 1955 to the XI, and so on. As the years went by, however, they realized that if Roman Numerals continued to be used they would require a very lengthy stamp: so, instead to indicate 1971 with the figure XXVII it was determined to indicate it with a mixed one of Roman numbers and Arabs: XX7. It follows XX8 for 1972 and XX9 for 1973, to end with XXX in 1974. From 1975, the Roman numbers were abolished and it was decided to indicate the year with a code of two letters. 1975 corresponds to AA, 1976 to AB, and so on. The problem is that some letters were skipped, therefore only having the entire list of the codes or proof marks you can determine with certainty the year of construction of the arm. Here is the entire list of the codes:

Then click on the link: The bench of Test
Scroll down a bit. There is a list of inspectors and then a table with date codes from 1922 to 1999.

If I am understanding this correctly. A gun was submitted for proof. The proof house marks the gun received. They proof it. They mark it as proofed. They mark the date. They measure and weigh the barrels. They record all of this on the barrels or the flats. Finally, I think the inspector marks the gun as completed. Also, the barrel maker may have their mark on the barrels and the gun maker may have their mark on it as well.